In this project the distribution of the nervous tissue in the lower respiratory system of the fetal and neonatal mice and human are investigated. The electron microscope is used as a major tool in conjunction with other histological and chemical techniques including histochemistry, fluorescent microscopy and surfactant analysis. The trachea, bronchi, bronchioles, alveoli as well as the pulmonary arteries and veins from mouse fetuses 10, 15, 17, 19 and 21 days old and from postnatal mice, 2, 5, 8, 12, 18 and 21 days old are studied. The tissues of the human lower respiratory system are obtained from autopsied fetuses and infants. Efforts are made to correlate electron microscopic observations with those of light microscopy and surfactant assay. The locations of the sensory and motor axons and their fine relationships with various cellular components of the developing respiratory system are investigated. The development of the sensory receptors and the adrenergic and cholinergic endings are followed.